The invention relates to an oxide cathode comprising a metal base and a heating element for heating said base, on which base a porous layer comprising an alkaline earth metal oxide is provided.
Such oxide cathodes are used in cathode ray tubes, for example display tubes for monochromatic and colour display of television pictures, camera tubes, storage tubes and oscillograph tubes.
Such an oxide cathode for a cathode ray tube is known inter alia from the article "Chemical Transport in Oxide Cathodes" Philips Res. Repts 26, 519-531, 1971. The oxide cathode described therein is a cathode of the so-called indirectly heated type which is composed of a base of polycrystalline nickel on which on one side a porous layer of alkaline earth metal oxides is provided. The other side is radiated by a heating element. The oxide layer generally has the composition EQU Ba.sub.x Sr.sub.1-x O
with x approximately equal to 0.5. The thickness of the layer is approximately 50 .mu.m and the density of the layer is approximately 0.7. The base comprises an activator, for example Mg, either in a solid solution or in regularly divided grains. Mainly BaO is reduced to Ba by said activator so as to obtain good emission properties which are characteristic of Ba on SrO. In this process, a diffusion along grain boundaries in the material of the base plays an important role.
An advantage of such an oxide cathode is the comparatively low operating temperature of approximately 800.degree. C. In television display tubes undesired grid emission is kept small by said comparatively low temperature. In camera tubes having a so-called diode electron gun the beam-discharge lag will be low due to said comparatively low temperature. Moreover, at such a low operating temperature, the power to be applied to the heating element will be smaller than in a cathode having a higher operating temperature. In order to avoid too large a Ba production and consequently evaporation of Ba at the beginning of the life of the cathode, the concentration of the activator in the nickel may be small. This means, however, that the base may not be too thin because in that case the activator would be exhausted too soon. Usually, therefore, the thickness is larger than 50 .mu.m and preferably is approximately 100 .mu.m. This puts a limit on the minimum warm-up time of the cathode. This is the time needed after switching on the voltage across the heating element for the cathode current to reach 10% of the steady state current supplied. In the case in which the operating temperature is 800.degree. C., the cathode temperature at 10% of the emission at the operating temperature is approximately 600.degree. C. For a 1.5 watt cathode used frequently in television display tubes the warm-up time is 5.5 seconds. Due to its comparatively large thickness together with the comparatively large specific heat and the comparatively large specific weight of the nickel, the base provides a considerable contribution to the overall heat capacity and hence to the warm-up time of said indirectly heated cathode. It is known that the warm-up time for directly heated cathodes may be considerably shorter than for the above-described indirectly heated cathode. A disadvantage of such directly heated cathodes is, for example, that cathode control cannot be used in a simple manner. Because the warm-up time is proportional to the quotient of the heat capacity and the stationary power supplied to the cathode, a smaller heat capacity of the base may be used to reduce the stationary power to be supplied if the warm-up time of the directly heated cathode is already sufficiently small. The base must go on fulfilling its BaO-reducing function for the required long life time and the adhesion of the porous oxide layer to the base must remain good.